The present invention generally relates to communication receivers, and particularly relates to the use of multiple scrambling codes in parametric Generalized Rake (G-Rake) receivers, such as the use of primary and alternate scrambling codes in a wireless base station transmitter.
Known receiver techniques for exploiting multipath reception in Code Division Multiple Access (CDMA) communication systems, such as cellular communication networks based on the Wideband CDMA (W-CDMA) standards, include the “Rake” receiver architecture. In operation, a Rake receiver assigns individual despreader circuits, often referred to as Rake “fingers,” to different path delays (taps) of a received multipath CDMA signal. A Rake combining circuit then combines the individual finger signals to form a combined signal having an improved signal-to-noise ratio if the finger signal noise is white.
Thus, the following equation represents Rake receiver operation with respect to a mobile station operating in a WCDMA network, assuming a multipath channel having d channel taps:
                                          H            C                    =                                    [                                                h                  0                  C                                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  h                                      d                    -                    1                                    C                                            ]                        T                          ⁢                                  ⁢                              H            D                    =                                    [                                                h                  0                  D                                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  h                                      d                    -                    1                                    D                                            ]                        T                                              Eq        .                                  ⁢                  (          1          )                    where HC represents the d×1 propagation channel vector for the Common Pilot Channel (CPICH), and HD represents the d×1 propagation channel vector for a Dedicated Physical Channel (DPCH) assigned to the mobile station.
With the channel vectors of Eq. (1), the below equation describes the received vector of despread signals for the Rake receiver:Y=HDu+E  Eq. (2)where Y=[yt, yt-τ1, . . . , yt-τd-1], HD is the DPCH channel vector as given in Eq. (1) and is assumed to be constant over at least NC CPICH pilot symbols, u is the transmitted symbol of interest, and E represents a noise vector having a diagonal covariance matrix given as:Σ=E(EEH)=diag(σ12, . . . , σd-12)  Eq. (3)where E(•) represents the expected value operation.
The Rake receiver uses the pilot symbols received on the CPICH to estimate the channel HC and the variance elements in Σ, and then scales Σ using the spreading factor difference between the DPCH and the CPICH. The following equations yield these estimates as follows:
                                              ⁢                                                            H                ^                            C                        =                                          H                C                            +                              E                                  H                  C                                                              ,                                    E                              H                C                                      ∈                          N              ⁡                              (                                  0                  ,                                                            SF                                              256                        ⁢                                                  N                          C                                                                                      ⁢                    Σ                                                  )                                                                        Eq        .                                  ⁢                  (          4          )                                                                            Σ              ^                        ii                    =                                                    σ                ^                            i              2                        =                                          256                SF                            ⁢                              1                                                      N                    C                                    -                  1                                            ⁢                                                ∑                                      k                    =                    1                                                        N                    c                                                  ⁢                                                                                                                        y                        k                        i                                            -                                                                                                    h                            ^                                                    C                          i                                                ⁢                                                  u                          k                          CPICH                                                                                                                          2                                                                    ,                  i          =          0                ,        …        ⁢                                  ,                  d          -          1                                    Eq        .                                  ⁢                  (          5          )                    where ĤC={ĥCi}, i=0, . . . , d−1 represents the CPICH channel estimate vector, SF is the spreading factor of the DPCH, and {circumflex over (σ)}i2 is normalized such that the expected value E({circumflex over (σ)}i2)=σi2.
Inserting the channel and interference estimates obtained from Eq. (4) and Eq. (5), respectively, in the Rake detector yields the following detector statistics for the Rake receiver:DRake=ĤCH{circumflex over (Σ)}−1Y  Eq. (6)From Eq. (6), one sees that the Rake receiver only estimates the propagation channel for the signal delays of interest and the corresponding interference (noise power) for each tap, as represented by the diagonal matrix Σ. Such operations yield optimal operation (maximum ratio combining) only if the finger signal noise is white. Those skilled in the art will appreciate that signal interference in WCDMA and other networks generally is not white because of same-cell and other interference terms.
More sophisticated receiver structures consider colored noise and other correlated signal impairments. Examples of such interference canceling receivers include chip equalizers and so-called “Generalized Rake” (G-Rake) receivers. Chip equalizers generally use a single received signal correlator, which is preceded by a whitening filter having a number of filter taps. With this arrangement, chip equalizers compensate for colored interference by calculating the filter tap weights at least partly based on measurements or estimations of colored interference in the received signal. Similarly, a G-Rake receiver suppresses colored interference by inserting additional “probing” fingers that are placed at delay positions not necessarily corresponding to tap delays of the multipath signal, and are used to estimate impairment correlations, including colored noise interference, for the on-path (data) finger signals.
Assuming a d tap channel with p probing fingers, the following equations define the CPICH and DPCH channel vectors, respectively:
                              H          C                =                              [                                                            h                  0                  C                                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  h                                      d                    -                    1                                    C                                            ,                              0                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                0                                      ]                    T                                    Eq        .                                  ⁢                  (          7          )                                                  H          D                =                              [                                                            h                  0                  D                                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  h                                      d                    -                    1                                    D                                            ,                              0                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                0                                      ]                    T                                    Eq        .                                  ⁢                  (          8          )                    Further, the received despread signal is given as,
                                          Y            =                                                            H                  D                                ⁢                u                            +                              E                ⁢                                                                  ⁢                where                                              ⁢                                          ⁢                                          ⁢                                    Y              =                              [                                                      y                    t                                    ,                                      y                                          t                      -                                              τ                        1                                                                              ,                  …                  ⁢                                                                          ,                                      y                                          t                      -                                              τ                                                  d                          -                          1                                                                                                      ,                                      y                                          t                      -                                              τ                                                  p                          0                                                                                                      ,                  …                  ⁢                                                                          ,                                      y                                          t                      -                                              τ                                                  p                                                      p                            -                            1                                                                                                                                              ]                                      ,                          ⁢                                                      Eq        .                                  ⁢                  (          9          )                    with the d delays representing path delays for the received signal of interest and with the p delays representing the probing finger delays. Further, as before, HD is the DPCH channel vector and is assumed to be constant over at least NC CPICH pilot symbols, u is the transmitted symbol of interest, E is the noise and interference vector, and R is an m×m impairment correlation matrix, which may comprise a covariance matrix. ({circumflex over (R)} equals the expected value E(EEH) and is normalized such that the expected value E({circumflex over (R)})=R.)
In turn, the G-Rake receiver estimates the channel HC and the covariance matrix R using the pilot symbols received on the CPICH, and then scales those estimates based on the DPICH-to-CPICH spreading factor difference. The following equations represent such estimation:
                                                        H              ^                        C                    =                                    H              C                        +                          E                              H                C                                                    ,                              E                          H              C                                ∈                      N            ⁡                          (                              0                ,                                                      SF                                          256                      ⁢                                              N                        C                                                                              ⁢                  R                                            )                                                          Eq        .                                  ⁢                  (          10          )                                                  R          ^                =                              256            SF                    ⁢                      1                                          N                C                            -              1                                ⁢                                    ∑                              k                =                1                                            N                C                                      ⁢                                          (                                                      Y                    k                                    -                                                                                    H                        ^                                            C                                        ⁢                                          u                      k                      CPICH                                                                      )                            ⁢                                                (                                                            Y                      k                                        -                                                                                            H                          ^                                                C                                            ⁢                                              u                        k                        CPICH                                                                              )                                H                                                                        Eq        .                                  ⁢                  (          11          )                    
With the above channel and interference estimates, the G-Rake detector statistic is given as,DG-Rake=ĤCH{circumflex over (R)}−1Y  Eq. (12)The above statistic in comparison with the Rake detector statistic of Eq. (6) makes clear that the G-Rake receiver compensates for impairment correlations as represented by the impairment covariance matrix, R. The R term can be estimated directly, such as from measured impairment correlations, or can be estimated parametrically, such as from modeled noise terms. However, the direct or indirect use of pilot symbols received on the CPICH generally remains a common feature of G-Rake colored interference estimation for DPCH signals.
That CPICH dependence generally is not problematic where the DPCH differs from the CPICH in terms of spreading factor but relies on the same base station scrambling code. However, conventional G-Rake (and, likewise, chip equalizer) estimations of DPCH colored interference may be inaccurate in instances where the transmitting base station uses different scrambling codes for the DPCH and the CPICH. As a non-limiting example, the WCDMA specifications allow a base station to transmit using more than one scrambling code, such as a primary and secondary (or alternative) scrambling codes. Such operation is useful when, for example, the primary scrambling code channelization tree is full but the base station has power capacity to serve additional users. In such cases, the base station transmits one or more such additional channels using one or more additional scrambling codes, meaning that interference estimates determined at a receiving mobile station may be inaccurate to the extent that the additional scrambling codes are not considered.
The use of dedicated pilot or other known symbols represents one approach to more accurate interference estimation for DPCH and other channels that use scrambling codes different from that used for the CPICH. However, the relatively low number of DPCH pilots transmitted in a given interval of interest compromise the performance of this approach and can leave the resulting interference estimates noise prone and poorly suited for use in fast fading channel conditions.